1. Technical Field
The present invention relates to an arrangement for transmitting torque of a main unit of a ship to oppositely rotating, double propeller shafts (generally called "contra-rotating propeller shafts) through a planetary gear train.
2. Background Art
As shown in FIG. 6 of the accompanying drawings, a propulsion system having contra-rotating propeller shafts for a ship includes two propeller shafts, namely an inner propeller shaft 105 and an outer propeller shaft 107, and two propellers 104 and 106 respectively mounted on the propeller shafts 105 and 107. The front propeller 106 is larger than the rear propeller 104 and they are spaced in the direction of the propeller shaft 105 by a predetermined distance. The inner propeller shaft 105 is coupled to an output shaft 102 of a main propulsion system 101 of a ship. The inner propeller shaft 105 is partially surrounded by a hollow shaft 109 of a sun gear 108 and the sun gear 108 is connected to the inner propeller shaft 105 via a clutch 110. The sun gear 108 meshes with a first planetary gear 111. The first planetary gear 111 is intergral with a second planetary gear 112 which is larger than the first planetary gear 111 in diameter. The second planetary gear 112 meshes with an internal gear 114. The internal gear 114 meshes with a gear 113 which is integral with the outer propeller shaft 107. In this manner, the inner propeller shaft 105 is drivingly connected to the outer propeller shaft 107 and the propeller shafts are roated in opposite directions.
When the ship is operated by two propeller shafts, chattering of the planetary gear set and stress of the inner propeller shaft must be taken into account. Combustion takes place in the main unit 1 (engine) and vibrations (generally tortional vibraions) due to the combustion are transmitted to the propellers 4 and 6. The vibration torque has its peak (a resonance point) at a natural frequecy of a power transmission line (the engine 1, the propellers 4 and 6 and other members) of the ship. FIG. 3 shows a case where a ship is propelled by two propellers and the power transmission line of the ship has a resonance frequency of 81.9 r/m (revolutions per minute). The dashed line A indicates a vibration torque, exerted on the gears and peripheral elements thereof the two-dot line B indicates an average torque exerted on the gears and peripheral elements thereof and the solid line C indicates a stress of the inner propeller shaft. When the vibration torque A is below the curve of average torque B, i. e., when the ship is operated at a speed beyond 93 r/m, the chattering does not occur. However, if the vibration torque A becomes greater than the average torque curve B (range D), the gears of the planetary gear set start chattering. The gears may be broken due to the chattering and therefore the gears cannot be used continuously. The stress C of the inner propeller shaft also has its peak at the natural frequency (81.9 r/m) of the power transmission line. In addition, the operation at a speed near 81.9 r/m (natural frequency) causes vibrations of the ship body and the engine. Therefore, it is not preferable to operate the ship at a speed near the natural frequency of 81. 9 r/m. In summary, with the chattering and the stress of the inner propeller shaft being considered, the ship have to be operated at a speed beyond 93 r/m as far as the two propeller are driven.
When the ship is approaching a harbor, for example, the speed of the ship is lowered. However, the ship should not be operated at a speed lower than 93 r/m as far as the two propellers 104 and 106 are driven. In this case, the outer propeller shaft 107 or the front propeller 106 is disconnected from the engine and the ship is driven by only the rear propeller 104. Upon the disconnection of the outer propeller shaft 107, the gears of the planetary gear set which would cause the chattering bear no load. Thus, the chattering does not occur at any speed. With respect to the stress of the inner propeller shaft 105, the natural frequency of the power transmission line is shifted to a higher value (from 81.9 to 98), as indicated by the line E in FIG. 4, since the inertia of the power transmission line is decreased. Then, no problem would occur even if the ship is operated at a speed below 93 r/m. Referring back to FIG. 6, the inertia of the power transmission line is changed by the disengagement of the clutch 110, i. e., the clutch 10 disconnects the shaft 9 of the sun gear 8 from the propeller shaft 5, whereby the inertia is lowered. MCR in FIGS. 3 and 4 stands for Maximum Continuous Revolution.
Meantime, in the arrangement of FIG. 6, the clutch 110 is provided on the inner propeller shaft whose diameter is large. A large torque is produced on the inner propeller shaft 102 as the propeller shaft 102 is rotated by the engine 101. Thus, a large torque is applied to the clutch 110 upon engagement of the clutch 110 as well as during propulsion torque transmission from the engine 101 to the outer propeller shaft 107. As a result, the conventional arrangement has to be very rigid and requires a clutch having a large capacity in terms of transmission torque.
In addition, time-consuming, troublesome work is necessary in the routine maintenance of the clutch 110. This is because it is necessary to remove a housing of the planetary gear set 103 and the planetary gears and then to move the propeller 104 and the inner propeller shaft 105 toward the end of the ship before the maintenance. Furthermore, since the conventionally employed clutch 110 is very large, as mentioned earlier, the maintenance is itself not easy.